Looks like a good solid performer from the schematic. All those LTP's - could include a balanced input option ?

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

I'm surprised they put such a good amp in a "mid-fi".
With my digital source(SB live -PC) , this really sounds wonderful on the Dan Richie audiophile 2-ways in the workshop.
(below link and attachment) X-LS Classic kit

The circuit - I've found that by loading the VAS (R25 -47K) you can control
the H2 / H3 balance in the Fourier results. 22k gives H2 dominant at the
expense of total THD. 33K seems to be the sweet spot , at least for these models.

The real amp uses 2sa992 /2sc1845 almost exclusively , 50VDC rails.
The longevity of this circuit is amazing .... found it in the college of Saint Rose dumpster
( many beer- soaked "enhancements" of "higher education" ).
It worked at lower volumes even with dry electrolytics -amazing.

The compensation is "weird" . I know C17 (lead comp.) and it's effect/purpose.
I'm curious about R7 /C5. I don't know what this network is called , but it greatly contributes to this amps stability.
The amp also exhibits the best clipping behavior , I would guess most of
32 years of clipping .

The Pioneer amplifier I moded into my TGM6 amplifier (there's a thread with schematics somewhere around here) which also has this 'R25', same value in fact. The load on the VAS was a key part of the compensation (like your R7 C5) as it also had no Cdom. The lead comp does seem a little large at 22pF, but not by much.

I love the speakers - red, great colour. I hope these weren't punished with 32 years of clipping !

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Ahhhh .... while others are asleep and fussing over the "revolution" , I
continue to dissect this cool circuit.

Attachment 1 shows the clipping behavior. At least it is symmetrical , but
shows a small amount of "sticking" . The edges are rounded , not as bad as
"blameless" clipping.

Attachment 2 is driving a capacitive load. I had to really coax this amp to
give me a damped ringing response. Bridged the OP inductor , lowered the
load resistor. VERY stable ... I've not simulated too many like this.

All this is reflected in the real amp , 32 years with original semi's is a pretty good track record .....
This must become a project !

The Pioneer amplifier I moded into my TGM6 amplifier (there's a thread with schematics somewhere around here) which also has this 'R25', same value in fact. The load on the VAS was a key part of the compensation (like your R7 C5) as it also had no Cdom. The lead comp does seem a little large at 22pF, but not by much.

I love the speakers - red, great colour. I hope these weren't punished with 32 years of clipping !

That was the "stock photo" of the AV123's , I have piano gloss black ones "made in Cali,Columbia"
(checked the boxes for contraband - found these
outside a local tavern - perfect condition).

I'm nice to my AV123's - I use them as a reference to hear the imaging and
subtleties of my amps , for clipping I use my "bombed out" realistic 3-ways
(10"/5"/3") - only $20 , even with new woofers.

I haven't done much amp stuff lately , I have been increasing my loudspeaker
"collection".
OS

Too many EF2 OPS's around here.
If I scale this up to a mje340/350 - njw0302/0281 - mjl21193/94 output stage,
perhaps builders will be a little less apprehensive of triples.

OS

Way too many. Especially when they parallel a dozen or so outputs and think they can drive an ohm. And then wonder why the VAS runs out of gas at normal current levels. I hate to run VAS's at 50mA because they burn up the PCB and drift all over the place. Much better to use EF3 with a couple miserable mA in the predriver.

A lot of builders have trouble with triples (tribbles?) because they insist on doing weird things with them. A straight complementary EF3 is pretty darn stable with either high or low fT parts in the driver/predriver.

Way too many. Especially when they parallel a dozen or so outputs and think they can drive an ohm. And then wonder why the VAS runs out of gas at normal current levels. I hate to run VAS's at 50mA because they burn up the PCB and drift all over the place. Much better to use EF3 with a couple miserable mA in the predriver.

A lot of builders have trouble with triples (tribbles?) because they insist on doing weird things with them. A straight complementary EF3 is pretty darn stable with either high or low fT parts in the driver/predriver.

I came to these same conclusions after reviewing some of the more popular
threads that use those EF2's with 5 or more OP's and claim 2R loads.

In the attempt to not "run out of gas " VAS wise , some are using drivers
to do the job. Besides the much higher Cob of these devices (mje15032/33 or
2sa1837/2sc4793 -not too bad) , you also have a limited choice of a 1A SOA drivers to run 5 or more pairs of "hungry" outputs.

Moon audio and other overpriced high-end amps use outputs as drivers. You can't (or shouldn't)
do this with the standard EF2.
I have the opinion that 3 pair of outputs is the maximum for the EF2.